越来越有感觉
unless在意义上相当于if…not…，因此在有些场合if…not…可为unless所代替。但在有些场合又不能互换，我们要注意下面几种情况：1.当if…not…引导非真实条件分句时，一般不可改用unless。 例如：If she had not had an alarm clock,she would have missedthe train.（事实：她有闹钟。）如果把上述例句中的if…not…换成unless，意义就相反了。（误）Unless she had had an alarm clock, she would havemissed the train.（事实：她没有闹钟）2.If…not…引导的条件分句可以重叠使用，unless则不可， 因为unless表示唯一条件。例如：She won't lose weight if she doesn't keep a diet and ifshe does not take exercise every day.3.unless引导的分句可用否定结构，而if…not …引导的分句不可再否定。例如：Don't ask me to explain unless you really don't understand.The directors have a meeting every Friday,unless there is nothing to discuss.以上两例句中的unless不可换成if…not…。4.在unless引导的肯定形式的条件分句中， 通常用肯定词（即some,already,still,too,as well等），在if…not…引导的分句中，通常用非肯定词（即any,yet,ever,either,at all等）。例如：I will criticize you if you haven't spoken to her yet.I will criticize you unless you have already spoken to her.（误）…unless you have spoken to her yet.不过在否定形式的非真实条件分句中，if…not …结构仍能与肯定词连用。例如：If he hadn't already prepared his lesson, he wouldn't beallowed to go.（因为这种从句表面上是否定，实际上是肯定。）5.在间接引语中，当if=whether，意为“是否”，unless 不可以代替if…not…。例如：She promised to let me know if she wasn't coming.6.当unless含有“除了”的意思，不可为if…not…所替代。例如：He never stammers,unless when he is angry.( unless when=except when)Nothing will come out of it, unless disaster.(unless=except)7.当主句是疑问句时，不可用unless代替if…not…。 因为unless用在A情况发生，必导致B结果之类的复合句中。例如：What shall we do it they don't reply to our letters?8.当主句谓语是be＋某些形容词或分词，如：glad,surprised,disappointed,satisfied,ashamed,delighted和pleased等， 不可用unless代替if…not…。例如：I'll be really surprised if they don't come to the meeting.I'll be quite glad if she doesn't join them.之所以不能为unless代替，是强调否定条件的出现，表明在这种前题下的因果关系。以上条件状语从句都可变为原因状语从句。例如：I'll be really surprised that they won't come to the meeting.这里if被that代替了，而从句的否定结构不变。综上所述，unless既等于if…not…，在某些场合又不相等， 因此要牢记它们之间的差别，准确地使用它们。
dongdong88z
一、词性： * if not例如: We will achieve many goals if we make good use of our time, if not we will get lost.如果我们好好利用时间我们就可以实现许多目标，否则 我们会迷失自我.1. 副词短语，意思是“不然的话, 否则” ，如： We should do it with all our heart. If not we will leave it unfinished. 我们应该全心全意地做这件事，不然的话，我们将会半途而废。 2. 连词，引导否定的条件状语从句，如： If we are not healthy, happiness is impossible. 没有健康就我们就不可能有幸福. * but for 否定复合介词，用在名词前构成介词短语，意思是“要不是”，等于 without。还可以扩展成 if ... not / unless 从句（见下）。 二、用法： * 连词 if not 用于条件句可以用真实语气，也可以用虚拟语气，如： 1. 真实语气： If he does not study hard, he will never pass the examination. 他如果不努力学习, （将来）考试就永远不会及格。 This baby seldom cries if he is not tired. 这个婴儿除非在疲倦时（往往）很少哭。 If I do not understand what he says, I always ask him. 要是（平时）我不明白他所说的，我总是问他。 2. 虚拟语气： How could I find you here if you were not at home? 要是你不在家，我怎么会在这里找到你呢？（对现在虚拟）. If he had not told me the truth about her, I would not have uderstood what she had said. 他假如没有告诉过我她的真相，我就不会明白他所说过的话。（对过去虚拟） * 但是，句子中只要有 but for，该句子必须用虚拟语气。 But for your help （if you had not helped us）we should not have finished in time. 要不是你的帮助，我们就不会及时完成。 But for the rain (= If it had not rained) we should have had a pleasant journey. 要不是下雨，我们就会有一个愉快的旅途的。
19870629侠女
1.当if…not…引导非真实条件分句时，一般不可改用unless。 例如：If she had not had an alarm clock,she would have missedthe train.（事实：她有闹钟。）如果把上述例句中的if…not…换成unless，意义就相反了。（误）Unless she had had an alarm clock, she would havemissed the train.（事实：她没有闹钟）2.If…not…引导的条件分句可以重叠使用，unless则不可， 因为unless表示唯一条件。例如：She won't lose weight if she doesn't keep a diet and ifshe does not take exercise every day.3.unless引导的分句可用否定结构，而if…not …引导的分句不可再否定。例如：Don't ask me to explain unless you really don't understand.The directors have a meeting every Friday,unless there is nothing to discuss.以上两例句中的unless不可换成if…not…。
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